This invention relates to a ringing signal detector which enables discrimination between a ringing signal to a terminal equipment from a communication circuit and a dial pulse signal from a telephone set connected to the communication circuit in parallel to the ringing signal detector.
A conventional ringing signal detector employed in a terminal equipment of a communication circuit comprises an exchange I, a telephone set II, a communication circuit III and a ringing signal detector IV. The ringing signal detector IV comprises a DC blocking capacitor for blocking a DC voltage superimposed on the communication circuit III, a DC converter for converting an AC ringing signal of 16 Hz to a DC voltage and a DC level detector for detecting the ringing signal with a change in the output potential from the DC converter. The ringing signal detector IV detects the ringing signal to start a terminal equipment. With such an arrangement, there is the likelihood of erroneous starting of the terminal equipment as follows: Namely, when a call is originated from the telephone set II connected to the communication circuit III in parallel to the terminal equipment, a dial pulse signal from the dial of the telephone set II is also applied to the terminal equipment and is detected by the ringing signal detector IV, as in the case of the ringing signal, to start the terminal equipment. In a case where a terminal equipment of the type performing automatic answering is connected, upon the starting, a DC loop of the communication circuit III is set up to prevent subsequent dialing, resulting in a serious trouble in that the telephone set II cannot originate a call.
One conventional system of preventing such malfunction is to set the detecting level of the ringing signal detector to be higher than a DC voltage resulting from the application of the dial pulse signal so that the detector will not operate in response to the dial pulse signal. With this system, however, since the states of the exchange I in the central office, the communication circuit III and the dial pulse signal generated by the superimposed DC voltage do not remain constant, the detecting level is difficult to set and must be set to be high. In such a case where the voltage of a ringing signal is low or several telephone sets II are connected in parallel, the ringing signal cannot be detected.
Another system is to prevent the malfunction by the employment of a monitor relay connected in series to the telephone set II while the contact of the monitor relay is connected in series to the DC blocking capacitor. With this system, when the telephone set II is put in the offhook state, the monitor relay operates to open the contact to disconnect or make the ringing signal detector IV of the terminal equipment inoperative, thereby ensuring prevention of the malfunction resulting from the application of the dial pulse signal. Since this system enables complete prevention of such an erroneous operation attendant with dialing, sensitivity for detecting the ringing signal can be enhanced. However, this system has defects in that it is expensive because of the use of the monitor relay and that since the connection of the telephone set II must be changed for the series connection thereto of the monitor relay, the system is not employed in the terminal equipment connected in parallel with the telephone set II.